In traditional optical systems having reflecting and refracting surfaces, it is common to assume that the light passing through the system is limited to paraxial rays, specifically, rays that are near the optical axis and are sustained within small angles. However, practical optical systems rarely are limited to paraxial rays, and thus, the actual images assumed by gaussian optics often depart from the xe2x80x9cperfectxe2x80x9d image. This departure from the xe2x80x9cperfectxe2x80x9d image results in the introduction of distortion into the optical system, called aberrations. These aberrations are most problematic in small focal ratio optical systems where the angle from the optical axis is larger.
In a monochromatic optical system with only spherical surfaces, there are five (5) basic ray and wave aberrations, namely, spherical aberrations, coma, astigmatism, curvature of field, and distortion. Optical systems for use with multi-chromatic light have an additional source of distortion, namely, chromatic aberration.
Because the distortion introduced by aberrations into an optical system significantly degrades the quality of the images on the image plane of such system, there are significant advantages to the reduction of those aberrations. Various techniques are often used to minimize the aberrations. For example, in order to minimize spherical aberrations or coma, a lens may be xe2x80x9cbentxe2x80x9d to have different radii of curvature on opposite sides while maintaining a constant focal length, such as is contemplated by using the Coddington shape factor. Also, a pair of lenses, where one glass lens has a positive focal length, and the other made from a different glass has a negative focal length, are used together to correct spherical aberrator. One example of this technique is the xe2x80x9cdoubletxe2x80x9d lens in which the two lenses have the same radius of curvature on the facing sides, and are cemented together.
Despite the available techniques to minimize the various aberrations, it is often difficult to simultaneously minimize all aberrations. In fact, corrections to an optical system to minimize one type of aberration may result in the increase in one of the other aberrations. Typically, one may decrease coma, at the expense of increasing spherical aberrations. Moreover, because it is often necessary to measure the aberrations only after an optical system is constructed due to additional aberrations from manufacturing or assembly tolerances, the creation of an optical system with minimal aberration typically requires several reconstructions before a suitable system is developed.
In complex optical systems, in addition to traditional aberration correction, it is often advantageous to create an optical element which generates a unique wavefront phase profile. Typically, these unique optical elements have been created by sophisticated grinding and polishing of traditional lenses. However, this method of manufacturing a unique optical element requires a significant amount of time and expertise, and results in a high cost of manufacturing the optical element.
Consequently, a need exists for the creation of an optical element which can generate a unique wavefront phase profile, and that can simultaneously minimize the chosen aberrations within an optical system.
The wavefront aberrator of the present invention includes a pair of transparent windows, or plates, separated by a layer of a monomers and polymerization initiators, such as epoxy. This epoxy exhibits a variable index of refraction as a function of the extent of its curing. Curing of the epoxy may be made by exposure to light, such as ultraviolet light. The exposure to light may be varied across the surface of the epoxy to create a particular and unique wavefront retardation profile such that when an ideal plane wave passes through the wavefront aberrator, a predetermined change of the wavefront profile can be affected by the wavefront aberrator device. Conversely, if a distorted wavefront is known, such as by measuring the wavefront with a Hartmann/Shack sensor, a correction of such aberrated or distorted wavefront aberration may be achieved by first producing a complementary wavefront aberrator device such that passing the abnormal wavefront through the wavefront aberrator device, a plane wave emerges.
One method of creating the wavefront aberrator of the present invention includes the exposure of the epoxy to an array of light emitting diodes (LEDs). These LEDs may be selectively illuminated such that different portions of the epoxy are exposed to different levels of illumination. This variance in illumination results in the creation of a wavefront aberrator having a varying index of refraction across its surface, and may include the formation of multiple sub-regions, where the index of refraction of the cured epoxy in a sub-region has a constant index of refraction, with the index of refraction varying between adjacent sub-regions.
An alternative method of creating the wavefront aberrator of the present invention includes the exposure of the epoxy to an array of LEDs through a demagnifier lens. In this manner, the LEDs may create a curing pattern which is then focussed onto the surface of the epoxy to create a similar, yet smaller, version of the curing pattern to provide for reduced-sized wavefront aberrators.
Yet another alternative method of creating the wavefront aberrator of the present invention includes the creation of a curing pattern by the transmission of light through a liquid crystal display (LCD). A non-coherent light source may be positioned adjacent to a diffuser to create a diffused light source. This diffused light may then be transmitted through a LCD containing a curing pattern, and onto a wavefront aberrator. As the epoxy is exposed, the curing pattern on the LCD creates the desired refractive index profile. New patterns may be generated by changing the pattern on the LCD.
A sensor may be placed beneath the wavefront aberrator to monitor the transmitted image of the curing pattern. The output of this sensor may be used to actively modulate the transmission of light through the LCD to create a wavefront aberrator having a desired refractive index profile, and to provide for an active monitor and control of the curing of each sub-region of the wavefront aberrator.
Another alternative method of creating the wavefront aberrator of the present invention includes the creation of a curing pattern by the selective illumination of portions of the epoxy using a point light source, such as a laser. This selective illumination may be accomplished by rastering a portion of the surface of the epoxy, varying the speed and/or intensity of the light source to vary the curing of the epoxy. Alternatively, the light source could trace particular curing patterns directly onto the wavefront aberrator at various speeds and/or intensities of light, such as by raster or vector scanning the curing pattern onto the aberrator. Also, a positive or negative, or xe2x80x9ccontact print,xe2x80x9d containing a particular wavefront retardation design may be positioned adjacent the wavefront aberrator and exposed to a diffused or collimated light to create the desired refractive index profile.